This study has a two-fold objective, one being to determine the biochemical aspects of virus-cell envelope interaction and the other being to increase our knowledge of the biochemistry of the cell surface itself. This objective will be pursued through the use of a model system consisting of T4 bacteriophage as virus and Escherichia coli as host. The particular advantages of this model system are described within. In this study, experiments will be performed to measure the effects on the cell envelope of T4 phage during the first stage in infection, viral adsorption, and also during subsequent stages of infection when specific phage genes affecting the cell envelope are expressed. In addition, because T4 perturbs the E. coli cell surface, the virus will serve as a probe for gaining more information on the E. coli cell surface itself. The starting point of this investigation will be a recently discovered phenomenon, namely the rapid release of cell envelope material which occurs upon adsorption of T4 to the E. coli cell surface (Loeb, 1974). Specifically, the experiments will be used to determine (1) the biochemical composition of the released envelope material, (2) the identity proteins on the exposed surface of E. coli and the changes these undergo during infection, (3) the role of T4 phage-mediated release of envelope material in the adsorption process, (4) the nature of the action of specific phage genes which bring about those changes in the envelope responsible for superinfection exclusion, immunity and resistance to lysis-from-without.